1. Field of the Invention
The present invention relates to an image forming apparatus capable of measuring a measurement image formed on a recording sheet.
2. Description of the Related Art
In an image forming apparatus, the quality of an image (hereinafter, referred to as an “image quality”) is determined based on graininess, in-plane uniformity, character quality, and color reproducibility (including color stability). In the recent spread of the multi-color image forming apparatus, the color reproducibility is sometimes referred to as the most material factor for determining the image quality.
Each person has a memory of colors (e.g., specifically, colors of human skin, blue sky, and metal) he expects based on his experience. The person will have uncomfortable feeling when seeing a color beyond a permissible range of the color he expects. Such colors are called “memory colors”. The reproducibility of the memory colors are often expected when photographs are output.
A demand for good color reproducibility (including color stability) is increasing with respect to the image forming apparatus. For example, in addition to the photo-images, there are office users who have uncomfortable feeling of difference in colors between a document image on a monitor and an actual document, and graphic art users who pursuit the color reproducibility of a computer-generated (CG) image.
To satisfy the good color reproducibility demanded by the users, for example, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus for scanning a measurement image (i.e., a patch image) formed on a recording sheet by using a measurement unit (i.e., a color sensor) provided in a conveyance path for conveying the recording sheet. With the image forming apparatus, density, gradation, and a tint can be reproduced to some extent such that feedback is given to process conditions such as an amount of exposure and a developing bias based on the scanning result of the patch image scanned by the color sensor.
When using a color management technique such as the International Color Consortium (ICC) profile, a measurement by white backing is a mainstream of the color management technique, and thus the International Organization for Standardization (ISO) 13655 defines regulations regarding the white backing. In the measurement by the white backing, the measurement is carried out such that a pressing member such as a white reference plate is pressed against a color sensor side from a back side of a recording sheet on which a patch image is formed to thereby measure the patch image with the color sensor. The measurement method can prevent the recording sheet from being fluttered while the recording sheet is conveyed and can keep a distance from the color sensor to the patch image on the recording sheet constant. Accordingly, a high precision measurement can be realized.
However, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-086013 has no pressing member such as a white reference plate at a position opposite the color sensor, so that the high precision measurement cannot be carried out with respect to the patch image.
In a case where the measurement is carried out with the white reference plate being pressed against a measurement object from a back side thereof according to the ISO13655, the high precision measurement can be carried out with respect to the patch image. However, on the other hand, since an attachment/detachment operation of the white reference plate takes time, the productivity is lowered.
On the other hand, in a case where the measurement is carried out without pressing the white reference plate against the measurement object from the back side thereof, the attachment/detachment operation of the white reference plate is no longer needed. Therefore, the productivity would not be lowered but, instead thereof, the measurement accuracy of the patch image is degraded.